As a conventional discharge lamp ballast apparatus for turning on a discharge lamp such as an HID (high-intensity discharge) lamp with a DC voltage, there is an apparatus that supplies a steady ballast voltage to the discharge lamp from a DC power source circuit; causes a starter to generate a one-directional high-voltage pulse for starting (referred to as a “start pulse” from now on) opposite in direction to the steady ballast voltage; and produces a dielectric breakdown between the two electrodes of the discharge lamp by superimposing the start pulse on the steady ballast voltage, thereby starting the discharge lamp. Superimposing the start pulse opposite in direction on the steady ballast voltage is carried out because it produces good experimental results (see Patent Document 1, for example).
As another conventional discharge lamp ballast apparatus, there is an apparatus that turns on the discharge lamp such an HID lamp by applying a square wave voltage. The apparatus has a DC power source circuit, an inverter circuit, a start signal output circuit, and a start pulse generating circuit, and turns on the discharge lamp by periodically inverting the DC voltage output from a DC power source circuit through an inverter circuit. To start the discharge lamp, the start signal output circuit detects the operation of the inverter circuit, drives the start pulse generating circuit at a time when a high-potential voltage is applied from the inverter circuit to the discharge lamp, and starts the discharge lamp by superimposing the start pulse output from the start pulse generating circuit on the pulse voltage output from the inverter circuit. Thus superimposing the start pulse on the high-potential pulse voltage and applying it to the discharge lamp facilitates the dielectric breakdown between the electrodes (see Patent Document 2, for example).
As for a discharge lamp turned on by a DC voltage, it has an anode and a cathode separately, and it sometimes becomes blurred or fragile because of the adhesion of ions to silica glass near the cathode. As still another conventional discharge lamp ballast apparatus, there is an apparatus that places a proximity conductor near the cathode of the DC discharge lamp, and maintains the proximity conductor at the same potential as the cathode of the DC discharge lamp, thereby preventing the fogginess. The discharge lamp ballast apparatus has a pair of start circuits having different application direction or application polarity of the high-voltage pulse at the anode side and cathode side of the DC discharge lamp, respectively. To start the DC discharge lamp, the individual start circuits supply the DC discharge lamp with positive start pulses and negative start pulses which are switched at random, thereby starting the discharge lamp surely and immediately. Thus applying the positive start pulses and negative start pulses at random to the DC discharge lamp makes it possible to improve the starting characteristics by starting the discharge by the negative start pulses when the DC discharge lamp is cold, and by the positive start pulses when restarting it (see Patent Document 3, for example).
Patent Document 1: Japanese patent publication No. 2-54639/1990 (pages 2 and 3, and FIGS. 3-8).
Patent Document 2: Japanese patent application laid-open No. 5-266984/1993 (pages 3-5, and FIG. 1)
Patent Document 3: Japanese patent application laid-open No. 10-241874/1998 (pages 4 and 5, and FIGS. 1-3)
With the foregoing configurations, the conventional discharge lamp ballast apparatuses have a problem of requiring a large amount of energy to start the discharge lamp. This is because they start the discharge lamp by applying the start pulse between the electrodes without considering the conditions between the electrodes of the discharge lamp, and hence the use efficiency of the start pulse is low.
The present invention is implemented to solve the foregoing problem. Therefore it is an object of the present invention to provide a discharge lamp ballast apparatus capable of starting the discharge lamp efficiently by applying a start pulse that will make a potential gradient steep around the electrode for emitting electrons.